Solar energy is an abundant resource that can provide a source of electrical energy. One method to collect and utilize solar energy is through photovoltaic (PV) cells, which convert solar energy directly into electrical energy. The conversion of energy can be provided by pn-junction diodes fabricated from n-type and p-type conductive regions in semiconductor materials. Such pn-junction diodes produce a photo-current generated when electron-hole pairs are formed in the semiconductor material. These electron-hole pairs are formed as a response to photons of electromagnetic radiation impinging on, and being absorbed within, the photovoltaic cell.
The energy absorbed by a semiconductor material is dependent on its characteristic band gap energy. The band gap energy of a semiconductor material may be defined as the amount of energy required to free an outer shell electron from its orbit about the nucleus to a free state. In semiconductor materials, the required energy for an electron to be excited from the valence band to the conduction band differs based on the energy difference between the two electronic states. Different materials have different characteristic band gap energies. Band gap engineering is the process of controlling the band gap energy of a material. Conventional silicon-based semiconductor materials used in photovoltaic cells (PVs) have a band gap energy of approximately 1.1 eV, which only covers a small portion of the solar electromagnetic radiation spectrum (e.g., from about 0.4 eV to 4.0 eV).
Photons of electromagnetic radiation having energy below the band gap energy of the semiconductor material will not be absorbed and converted to electrical energy. Photons with energy above the band gap energy may be absorbed, but electron-hole pairs that are created may quickly lose their excess energy above the bandgap energy in the form of thermal energy (i.e., heat). Thus, this excess energy is not available for conversion to electrical energy.
In order to maximize the absorption and the conversion of energy in a photovoltaic cell, multi junction (MJ) photovoltaic devices have been developed. Multi junction photovoltaic devices are made up of two or more subcells, each subcell comprising a pn-junction diode with a different characteristic band gap energy. Thus, each of the subcells has a band gap energy engineered to absorb different wavelengths of electromagnetic radiation within the solar spectrum. The two or more subcells can therefore absorb energy from different portions of the solar energy spectrum, resulting in better utilization of the solar energy and a higher operational efficiency.
Multi junction photovoltaic cells are commonly fabricated from two or more subcells formed as a vertical stack in a layer-by-layer deposition process. Each subcell is designed to absorb and convert a different portion of the solar energy spectrum than that of an adjacent subcell. The subcell first exposed to radiant energy generally has the highest band gap energy, while subcells positioned below the first subcell have correspondingly smaller band gap energies. As a result of this arrangement, energy that is not absorbed in the first subcell may be transmitted to, and converted to an electron-hole pair within, another underlying subcell of the multi junction photovoltaic device, thereby enabling a broader spectrum of solar energy to be converted into electrical energy.
However, common multi junction photovoltaic devices are limited due to the necessity to match the crystal lattice of each of the subcells during the sequential heteroepitaxial growth of the various layers of different materials over one another. Lattice mismatch between the crystal lattices in the different materials of adjacent subcells can result in mechanical strain and lattice dislocations (i.e., defects in the crystal structure) that reduce the efficiency of the photovoltaic device. As a result, materials that may be used in a typical multi junction photovoltaic cell are limited due to such lattice matching constraints.
To overcome the restriction imposed due to lattice matching concerns between two or more subcells of the multi junction photovoltaic cell during epitaxial growth processes, a bonding process may be utilized. Bonding of two or more subcells together allows a further degree of freedom in the selection of semiconductor materials that can be used to form a multi junction photovoltaic cell. The lattice mismatch may be overcome by forming two or more subcells on different substrates and then attaching the two or more subcells to one another by a bonding process.
The attachment of two or more elements is commonly performed utilizing bonding techniques. Such bonding techniques encompass a number of methods commonly referred to as, for example, molecular bonding, fusion bonding, metallic bonding, adhesive bonding, solder bonding, and direct bonding. For example, see the journal publications of Tong et al., Materials, Chemistry and Physics 37 101 1994, entitled “Semiconductor wafer bonding: recent developments,” and Christiansen et al., Proceedings of the IEEE 94 12 2060 2006, entitled “Wafer Direct Bonding: from Advanced Substrate Engineering to Future Applications in Micro/Nanoelectronics.”
The bonding of elements to one another is commonly assisted by the formation of one or more bonding layers on a surface of at least one of the elements to be bonded. The surface chemistry of the bonding layers can be manipulated to improve the adhesion of the two elements to one another, such that the two elements can be attached with sufficient bonding energy to enable further processing to be performed on the bonded semiconductor structure without premature separation.
Bonding layers may encompass a multitude of materials including, for example, conductors (e.g., metals), semiconductors and insulators. One of the more common bonding layers comprises a silicate such as, for example, silicon dioxide, wherein the surface chemistry of the silicon dioxide surface may comprise silanol (Si—OH) groups capable of producing high bonding energies. However, the use of insulating bonding layers may prevent the flow of electrons between the bonded elements, such as the subcells making up a multi junction photovoltaic cell. As a result, such bonding layers may not be suitable for use in devices where electrical conduction across the bonding layer is required.
The flow of electrons and, hence, electrical current between two bonded elements can be realized by utilizing metallic bonding layers. Metallic bonding layers have been produced using a number of different metallic materials such as, for example, copper and gold. However, the use of metallic bonding layers can severely limit the transmission of light through the bonded structure since metallic bonding layers may substantially prevent light transmission when the metallic layers are above a certain thickness. Therefore, metallic bonding layers may not be suitable when light transmission through the bonded elements is necessary.